Traditionally, mobile communications networks are arranged in a homogeneous structure, with the network comprising base stations (also known as Node Bs) arranged in a planned layout in which all base stations have similar transmit power levels, antenna patterns, receiver noise floors, and similar backhaul connectivity to the data network. Moreover, all base stations offer unrestricted access to consumer mobile devices (also known as User Equipments—UEs) in the network, and serve roughly the same number of mobile devices. Current wireless systems falling under this category include, for example, GSM, WCDMA, HSDPA, LTE and WiMAX.
More recently, heterogeneous mobile communications network structures have been considered. Heterogeneous networks are an efficient network deployment solution for satisfying the ever-increasing demand of mobile broadband services. In a heterogeneous network, a low- or lower-power node (LPN), small cell, for example a picocell, microcell or femtocell base station (NodeB), is placed in a traffic hot spot or coverage hole within the coverage area of a high- or higher-power node, for example a macrocell base station, to better serve nearby user equipments. Deploying a low power node in a traffic hot spot may significantly reduce the load in the macro or other higher-power cell covering the area.
FIG. 1 shows an exemplary heterogeneous UMTS mobile communication network 100 that comprises a macrocell node/base station (NodeB) 101 that establishes a cell with a coverage area (or cell) 103. Two low power nodes/base stations 105, 107 (for example microcell base stations) are located within the coverage area 103 of the macrocell node 101, each defining a respective coverage area 109, 111.
A user equipment 113 is shown in the cell 109 of low power node 105 and the cell 103 of the macrocell base station 101.
With increasing demand for such networks, power consumption is of increasing concern, with greater demands on reducing the power consumption of such networks whilst increasing the capacity.
Small cell and macrocell energy efficiency varies depending upon the population density and the number of subscribers. In most cases, small cells require significantly less power than macro cells on a kwh/Minute of Use (MOU) comparison.
As a rule of thumb, the smaller the cell the more power efficient it is on a per user basis. The average macrocell will utilize around 1000 W of energy to serve a maximum of around 120 simultaneous users and is more power efficient in heavily populated areas compared to sparsely populated rural areas. In comparison, a home femtocell typically and more efficiently requires 20 mW of power and accommodates between 4-8 users, and an enterprise femtocell will require 200 mW of power for 60 simultaneous users.
Indoor small cells are able to run on a lower transmit power compared to macrocells because the radio signal originates within a matter of feet from the user equipment. In addition, the user and the equipment are both indoors, so the RF signal does not need to use excess energy to penetrate through the outer wall and saves additional RF power consumption. Small cells use less RF power per user than macrocells in an mW of RF power per user comparison.
Within such networks, user equipment will select a cell (camp on a cell) which is most suitable to enable communication. For mobile equipment, the user equipment 18 may perform soft handovers from one small (low power) cell 109,111 (small cell) to a macrocell 103 or another of the small cells 109,111. The selection of a suitable cell is based on satisfying criteria which is defined for GERAN A/Gb mode or GERAN Iu mode in 3GPP TS 43.022 3GPP TS 43.022 for the GSM/EDGE radio access technology, for Universal mobile telecommunications system Terrestrial Radio Access Network (UTRAN) in 3GPP TS 25.304 3GPP TS 25.304 for the Universal Mobile Telecommunications System (UMTS) radio access technology and for E-UTRAN in 3GPP TS 36.304 3GPP TS 36.304 for the Evolved UTRAN (E-UTRAN) radio access technology.
PEMAX (or p-Max) is a key parameter in all cell selection processes. It represents the Maximum transmit power level a UE may use when transmitting on the uplink in the cell (dBm) defined as PEMAX (or p-Max) in 3GPP TS 36.301.
The operator determines the maximum cell radius by limiting the maximum TX power level, a UE can use (p-Max).
Initial Cell Selection process and possible transitions thereafter are detailed in 3GPP TS 36.304 for the E-UTRAN radio access technology.
When a UE is camped normally in RRC_IDLE mode, the UE performs the following tasks: select and monitor the indicated Paging Channels of the cell according to information sent in system information; monitor relevant System Information (SI); perform necessary measurements for the cell reselection evaluation procedure; execute the cell reselection evaluation process on the following occasions/triggers: 1) UE internal triggers, so as to meet various performance requirements; 2) When information on the BroadCast CHannel (BCCH) used for the cell reselection evaluation procedure has been modified.
The UE monitors the Paging CHannel (PCH) to receive System Information change notifications in RRC_IDLE. Changes in the system information are indicated by the network using a Paging message. When the Paging message indicates system information changes then UE shall re-acquire all system information as specified in 3GPP TS 36.331.
When the existing cell reselection evaluation process is triggered, measurements are initiated on candidate cells based on defined cell reselection priority rules as indicated in System Information 3GPP TS 36.304 for the E-UTRAN radio access technology.
Assuming measurements are initiated, measurement data is collected and the following cell selection criteria is applied;
The cell selection criterion S is fulfilled when:Srxlev>0 AND Squal>0whereSrxlev=Qrxlevmeas−(Qrxlevmin+Qrxlevminoffset)−PcompensationSqual=Qqualmeas−(Qqualmin+Qqualminoffset)                where:        
SrxlevCell selection RX level value (dB)SqualCell selection quality value (dB)QrxlevmeasMeasured cell RX level value (RSRP)QqualmeasMeasured cell quality value (RSRQ)QrxlevminMinimum required RX level in the cell (dBm)QqualminMinimum required quality level in the cell (dB)QrxlevminoffsetOffset to the signalled Qrxlevmin taken into account in theSrxlev evaluation as a result of a periodic search for ahigher priority Public Land Mobile Networks (PLMN)while camped normally in a Virtual PLMN (VPLMN)GPP TS 43.022 for the GSM/EDGE radio accesstechnologyQqualminoffsetOffset to the signalled Qqualmin taken into account in theSqual evaluation as a result of a periodic search for ahigher priority PLMN while camped normally in aVPLMN 3GPP TS 43.022 for the GSM/EDGE radioaccess technologyPcompensationmax(p-Max − PPowerClass, 0) (dB)PEMAXMaximum TX power level an UE may use whentransmitting on the uplink in the cell (dBm) defined as p-Max in [TS 36.101]PPowerClassMaximum RF output power of the UE (dBm) accordingto the UE power class as defined in [TS 36.101]
The signaled values Qrxlevminoffset and Qqualminoffset are only applied when a cell is evaluated for cell selection as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN 3GPP TS 43.022 for the GSM/EDGE radio access technology. During this periodic search for higher priority PLMN the UE may check the S criteria of a cell using parameter values stored from a different cell of this higher priority PLMN.
The cell reselection evaluation process does not differentiate between cell type (macro, micro, pico, etc.). As the cell selection process hinges on signal strength srxlev and fact the small cells will have lower srxlev compared to macro cells, even though there are suitable small cells, the UE will always select a suitable macro cell. This is poor use of the power gains which can be achieved if a suitable small cell is not given higher priority than a suitable macro cell in a macro-small cell network.